There is dopant technology as one of techniques for sensitizing silver halide photographic materials. The term “dopant technology” means a technique of incorporating materials other than silver ions and halide ions (dopants) into silver halide grains (doping technology). This technology allows transition metal ions to be introduced into the silver halide grains. Even though the amount of the transition metal ions introduced is extremely slight, compared with the amount of the silver ion constituting the silver halide grains, it has generally been observed that the transition metal ions effectively change the properties of photographic emulsions containing the silver halide grains doped.
In order to more enhance the sensitivity of the silver halide emulsion by the doping technology, it is most effective to dope the silver halide grain with not only a bare transition metal ion but also a “hexacyano complex” having a transition metal ion as a central metal and 6 ligands as cyanide ions. In particular, many examples of highly sensitized emulsions obtained by doping the silver halide grains with the group VIII metal ions are disclosed. Japanese Patent Publication No. 35373/1973 discloses hexacyanoferrate (II) complexes and hexacyanoferrate (III) complexes as dopants containing cyanide ions, but it discloses that the effect of high sensitization is limited to the case that iron ions are contained, regardless of the kind of ligand. However, it is shown that the use of cyanide ions as ligands is an effective means for obtaining highly sensitized emulsions, as described, for example, in Japanese Patent (Application) Laid-Open No. 66511/1993, U.S. Pat. No. 5,132,203, European Patents 0336425 and 0336426, and Japanese Patent Laid-Open No. 20854/1990. Central metals used in these disclosed examples include iron, rhenium, ruthenium, osmium and iridium. Further, an example of the use of cobalt (III) is disclosed in U.S. Pat. No. 3,790,390, and an example of the use of rhodium (III) is disclosed in U.S. Pat. No. 4,847,191. On the other hand, as highly sensitized emulsions obtained by doping with complexes having ligands other than the cyanide ions, U.S. Pat. No. 2,448,060 discloses that an emulsion doped with a complex of platinum (II) or palladium (IV) having a halogen ion as a ligand is sensitized, Japanese Patent Laid-Open No. 118535/1991 discloses sensitization by doping with a transition metal complex in which one ligand of a six-coordinated metal complex is carbonyl, and Japanese Patent Laid-Open No. 118536/1991 discloses that the internal sensitivity of an emulsion containing a transition metal complex in the inside thereof in which two ligands of a six-coordinated metal complex are oxygen increases. Further, examples of highly sensitized emulsions obtained by doping with complexes having organic compounds as ligands are also known. Japanese Patent Laid-Open No. 38249/1969 and U.S. Pat. No. 3,672,901 disclose examples of highly sensitized emulsions obtained by doping with iron complexes having organic compounds including [Fe(EDTA)]2− (EDTA=ethylenediaminetetraacetic acid) as ligands, and Japanese Patent Laid-Open No. 259749/1990 discloses examples of highly sensitized emulsions obtained by doping with [Fe(C2O4)3]3−. U.S. Pat. Nos. 5,360,712, 5,457,021 and 5,462,849, European Patent 0709724, and Japanese Patent Laid-Open Nos. 72569/1995 and 179452/1996 discloses examples of the use of complexes having many organic compounds as ligands. It is described that the effect of sensitization is significant particularly in doping with [(NC)5Fe(m-4,4′-bipyridine)Fe(CN)5]6−. In Japanese Patent Laid-Open No. 24194/1999, an emulsion high in sensitivity and improved in reciprocity failure is obtained by doping with [Fe(CO)4(P(Ph)3)]0 or [Fe(CO)3(P(Ph)2)]0. Further, according to Japanese Patent Laid-Open No. 102042/1999, an emulsion having high sensitivity is obtained when 2-mercaptobenzimidazole, 5-methyl-s-triazolo(1.5-A)pyrimidine-7-ol or 2-mercapto-1,3,4-oxadiazole is used as L in an [M(CN)5L]3− (M=Fe2+, Ru2+, Ir3+), [Fe(CO)4L]0, [M′(CN)3L]− (M′=Pd2+, Pt2+) or [IrCl5L]2− type complex. Furthermore, Japanese Patent Laid-Open No. 341426/1993 discloses emulsions to which some complexes including [Ru(bpy)2Cl2]0 (bpy=2,2′-bipyridine) are added. However, it discloses that the effect at the time when these complexes are added during the formation of grains is not so much, and that the significant effect is obtained by adding the complexes to grain surfaces together with dyes after the formation of grains.
In addition to the above, as examples in which transition metal complexes are added to grain surfaces, not as dopants, as described in Japanese Patent Laid-Open No. 341426/1993, to play a spectral sensitizing dye role, iron and molybdenum complexes of cyclopentadiene are each disclosed in U.S. Pat. No. 5,457,022 and German Patent 19629981, respectively. However, in either case, the inside of the grains is not doped with the complex, and the silver halide grains themselves are not allowed to have spectral (sensitizing) sensitivity. Examples in which silver halide grains themselves can have absorption bands in a wavelength region of visible light are described in Photogr. Sci. Eng., 28, 202 (1984) in which spectral sensitizing dyes are incorporated into silver halide grains, and Photogr. Sci. Eng., 25, 225 (1981) in which silver halide grains having spectral (sensitizing) sensitivity are obtained by coating PbO grains with AgCl. However, in these processes, the silver halide grains are obtained by the diffusion processor by coating of inorganic crystals with a silver halide, and these processes are largely different from the usual preparation processes of silver halide emulsions. Accordingly, such emulsions can not be easily used as actual silver halide photographic materials.